Groundwater is water located under the Earth's surface. It is considered to be the most precious of all geologic resources due to the fact that millions of people are dependent upon it for drinking, irrigation, and industrial purposes. Groundwater occurs naturally in the pores and fractures of rock and sediment. Although Water contained in some materials is largely immobile, water contained in other materials is capable of migration in response to a pressure gradient. Such reservoirs of groundwater are generally known as aquifers.
Groundwater in most aquifers has a slow rate of natural movement, generally less than about 4 cm/hr. The migration of groundwater is of interest for many reasons. For example, geohydrologists study the azimuth (compass direction) of the migration and its speed because they provide information on the subterranean formation itself. But it is in the environmental area that determination of the azimuth of groundwater migration is probably most critical.
When groundwater becomes contaminated at one place, the contamination moves to other places by transport down the hydraulic gradient. Accordingly, a contaminant source (e.g., a landfill or an accidental spill of a hazardous liquid) can be a threat to persons living great distances away who rely upon uncontaminated groundwater if the groundwater has a natural movement in their direction from the contaminant source.
Determining the azimuth of groundwater migration is a required step in predicting the environmental impact of a contaminant source. The standard method of determining the azimuth of migration in an aquifer is to take hydrostatic pressure readings at known depths in three widely separated boreholes. Using either trigonometry or graphic geometry, the pressure gradient and the azimuth of fluid migration can then be calculated. The azimuth of flow is traditionally assumed to be in the same direction as the pressure gradient. This method is not exact because it assumes homogeneity of permeability in the formation, and true homogeneity of a formation's physical character is rare. More importantly, this method of determining the azimuth requires three boreholes and the drilling of such boreholes is extremely expensive. Accordingly, a simpler, faster, less expensive, and more accurate method of determining azimuth would be a tremendous advance in this science.
A device to directly determine the azimuth of groundwater migration was at one time, and may still be, commercially available. The device contained a heating element surrounded by temperature sensors. The device was submerged in the groundwater and the heating element activated. By monitoring the increase in water temperature at the sensors, the azimuth could allegedly be determined. Unfortunately, the device was apparently not commercially successful. It is believed that the necessity of complication on-site calibrations and the sensitivity required because of the minute temperature differentials involved have prevented use of the device under rugged conditions.
The use of light sources to measure the velocity of a flowing liquid has been disclosed. Keller, U.S. Pat. No. 4,206,999, issued Jun. 10, 1980, discloses an apparatus which measures the direction and speed of flow of a fluid. The apparatus uses a rectangular laser beam which is aimed at the fluid. The beam is rotated until the light reflected from entrained particles is maximized. From this information, the velocity vector is determined.
Lord, U.S. Pat. No. 4,396,943, issued Aug. 2, 1983, discloses a video flowmeter. The flowmeter includes a television camera which takes movies of entrained particles moving with a fluid in a pipe. By adjusting the raster line scanning speed of the video screen, the images of the particles can be made to appear moving slowly across the video screen. The speed of movement is then compared with speeds for which a volumetric flow rate had been measured and a volumetric flow rate is determined.
Hasegawa, U.S. Pat. No. 4,543,834, issued Oct. 1, 1985, discloses a method of measuring a fluid velocity by forming bubbles in the fluid, illuminating the fluid with a slit of light, measuring the resulting diffusion as the bubbles pass at two close points a known distance apart, and then calculating the fluid velocity from the time interval between the appearance of diffusion at the two points.
Optical light sources have also been used to detect the speed and direction of moving objects. See, for example, Hock, U.S. Pat. No. 3,904,295, issued Sept. 9, 1975; Schneider, U.S. Pat. No. 4,286,872, issued Sept. 1, 1981; and Studer, U.S. Pat. No. 4,616,931, issued Oct. 14, 1986.
However, in summary, no known method or apparatus is capable of directly and reliably measuring the azimuth of groundwater migration.